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ICJ 9110 



Bureau of Mines Information Circular/1986 



Development of an Automated 
Breathing and Metabolic Simulator 



By Nicholas Kyriazi 




UNITED STATES DEPARTMENT OF THE INTERIOR 



(AXwi^ ■fefokv* , £iA/u^c^ 'khswJ 



Information Circular/ 9H0 



Development of an Automated 
Breathing and Metabolic Simulator 



By Nicholas Kyriazi 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 










Library of Congress Cataloging in Publication Data: 



Kyriazi, Nicholas 

Development of an automated breathing and metabolic simulator. 

(Information circular ; 9110) 
Bibliography. 

Supt. of Docs, no.: I 28.27: 9110. 

1. Respiration- Simulation methods. 2. Metabolism -Simulation methods. 3. 
Respirators -Testing. I. Title. II. Series: Information circular (United States. Bureau of 
Mines) ; 9110. 



TN295.U4 



[QP177] 



622 s [681'. 761] 



86-600287 



CONTENTS 

Page 

Abstract. 1 

Introduction 2 

Past developments 3 

IBM ABMS 3 

Reimers MBMS 5 

Bellows flexibility 8 

N 2 fidelity 8 

System response time 8 

The hypoxia scenario 8 

Reimers ABMS 9 

DEEC Inc. ABMS 11 

Conclusions 15 

Appendix. — Average inhaled gas values 16 

ILLUSTRATIONS 

1 . IBM ABMS photo 3 

2. IBM breathing and metabolic systems schematic 4 

3. IBM breathing-simulation system schematic 4 

4. IBM temperature and humidity system schematic 5 

5 . IBM electric furnace 5 

6. Reimers MBMS photo 6 

7 . Reimers MBMS schematic 7 

8. Reimers ABMS photo 9 

9. Reimers ABMS breathing-simulation system schematic 10 

10. Reimers ABMS temperature and humidity system schematic 11 

11. Reimers ABMS metabolic-simulation system schematic 12 

12. DEEC Inc. ABMS photo 13 

13. DEEC Inc. ABMS schematic 14 

14. Comparison of CO2 breath waveforms 15 

A-l. C0 2 and 2 breath waveforms 16 

A-2. Single inhalation air column 16 

A-3. Minimum C0 2 value vs average inhaled C0 2 value 17 



UNIT 


OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


h 


hour mln minute 


L 


liter ms millisecond 


L/min 


liter per minute yr year 


lb 


pound 






DEVELOPMENT OF AN AUTOMATED 
BREATHING AND METABOLIC SIMULATOR 



By Nicholas Kyriazi 1 



ABSTRACT 

The Bureau of Mines has been developing breathing and metabolic simu- 
lator technology since 1970. Breathing simulation has been widely 
achieved throughout the world and used in the testing of open-circuit 
breathing apparatus, but satisfactory metabolism simulation has not been 
achieved. This situation required that the testing of closed-circuit 
breathing apparatus, which are the only type used in mines, be done us- 
ing human test subjects. The goal was a machine that could accurately 
simulate both the breathing and the metabolic functions of a human being 
for testing of closed-circuit breathing apparatus. The advantages of 
using such a machine instead of a human being for testing respiratory 
protective devices lie in its ability to quantify metabolic input, its 
repeatability, and the lack of a need to deal with the vagaries of human 
subjects. This report will describe the breathing and metabolic simu- 
lators that have been developed and used by the Bureau over the past 
15 yr. 

"Biomedical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



Closed-circuit rescue breathing appara- 
tus have been used for over 75 yr in mine 
rescue and recovery missions after fires 
or explosions have made the mine atmos- 
phere irrespirable. Since 1981, closed- 
circuit escape breathing apparatus have 
been legislatively mandated for every 
person going into an underground coal 
mine in the United States. At present, 
testing of newly developed and exist- 
ing apparatus both by manufacturers and 
by the certifying agency, the National 
Institute for Occupational Safety and 
Health (NIOSH), is largely dependent upon 
human subject testing. Since metabolic 
demand varies with weight and condition 
of the human subject, and since even the 
same subject performing the same physical 
activity can have different metabolic 
demand levels depending upon posture and 
type of food recently eaten, it seems 
logical to question the fairness of quan- 
titative evaluations of breathing appara- 
tus based upon human-subject testing. 
This is the reasoning behind the research 
and development efforts by the Bureau of 
Mines in its quest for a simple, reli- 
able, and repeatable breathing and met- 
abolic simulator for the quantitative 
testing of breathing apparatus. 

The first concept utilized for metabo- 
lism simulation was that of burning pro- 
pane; this process simulated not only 2 
consumption but also CO2 production. 
While achieving a measure of success, 
this concept proved to be very compli- 
cated and maintenance intensive. Also, 
there is always a degree of danger pre- 
sent when burning a combustible gas. 

The second concept was that of cyclical 
removal and replacement of inhaled gases. 
On a breath-by-breath basis, gas is re- 
moved from the system to simulate 2 
consumption. Since simple removal of in- 
haled gas is not selective of 2 , some 
N 2 is also removed that must be re- 
placed on the reverse cycle. C0 2 is also 
added during the reverse cycle. A manual 



simulator of this type was used for 2 yr 
at the Bureau's Pittsburgh Research Cen- 
ter. The major problem with this concept 
lay in its extreme mechanical complexity. 
The automated version of this simulator 
also suffered from the same problem. 
In addition, the large internal volume 
of this second-generation simulator had 
the effect of slowing the system re- 
sponse to rapid changes in inhaled gas 
concentration. 

The presently used (third-generation) 
simulator utilizes the removal-replace- 
ment concept to simulate metabolism but 
does so on a continual basis rather than 
a cyclical one. In addition, all of the 
breathing and metabolic functions are 
controlled by a computer. The mechanical 
portion of the machine is simple; the 
complexity of the system lies in its com- 
puter software, which is not subject to 
maintenance problems. This concept was 
developed through a contract to the Noll 
Laboratory for Human Performance Research 
at the Pennsylvania State University and 
built by DEEC Inc. , a company formed by 
employees of the university who were 
involved with the contract. 

The third-generation automated breath- 
ing and metabolic simulator (ABMS) has 
the capability to vary over a wide range 
the following metabolic parameters: ven- 
tilation rate, 2 consumption rate, CO2 
production rate, respiratory frequency, 
tidal volume, breathing waveform, and 
breathing gas temperature. Also, any 
number of work rates may be combined in 
any order to simulate various activities. 
The simulator monitors numerous param- 
eters including average inhaled levels 
of 2 and C0 2 , breathing resistance, 
and inhaled gas temperatures. The test 
results are stored on either a floppy or 
a hard disk and may be plotted in any 
manner desired. 

The use of the Bureau-developed ABMS 
enables quantitative testing of closed- 
circuit breathing apparatus that are used 



in the mining industry for both escape 
and rescue. As mentioned, such apparatus 
are tested and certified by NIOSH using 
human subjects of various weights. This 
makes apparatus design difficult as manu- 
facturers do not know the weight of the 
subject their apparatus will be tested 
on. Present in human-subject testing are 
many unknown variables such as flow rate, 
exact C0 2 production, and exact 2 con- 
sumption, all of which vary with the 
weight of the test subject. Therefore, 
it is necessary to overdesign the 



apparatus to accommodate even the worst- 
case (heaviest test subject) situation. 
The Bureau will soon formally propose to 
NIOSH that a simulator such as the third- 
generation ABMS be used in certification 
testing of breathing apparatus in order 
to correct the perceived deficiencies in 
the present methods. 

In the following sections, more de- 
tailed descriptions are provided of the 
succession of Bureau -developed breathing 
and metabolic simulators. 



PAST DEVELOPMENTS 



IBM ABMS 

This simulator was developed by the 
Bureau through a research and development 



contract with IBM completed in 1973. It 
was extensively modified by the Bureau 
over the following years. (See figure 
1.) While it served its purpose during 




FIGURE 1.-IBM ABMS photo. 



To apparatus 
being tested 






Combustion 
chamber 




Water trap 


4 








t * 


v 


D— 


J ulse damper 
Flow sensors 


s — Q 


L> 








®— X — Metering valves — - Y — ® 


Dump 


Pump 






speed 
control 



- Solenoid valves - 



From 
CO2 supply 



From 
propane supply 



^Safety circuit 



FIGURE 2.— IBM breathing and metabolic systems 
schematic. 



those years, it was also very complicated 
and required continual maintenance. 

The metabolism simulation by this ma- 
chine was effected by the burning of 
propane that consumed 2 and produced 
C0 2 . (See figure 2.) The breathing sim- 
ulation was controlled by a variable- 
speed motor that was connected to a 
cylindrical, metal piston through linkage 
of a crankshaft, lever, and fulcrum. 
(See figure 3. ) The original lung was 
a flexible bellows that was replaced 
by the nonflexible, sliding-seal piston. 
The tidal volume could be changed by mov- 
ing the fulcrum. The functional residual 
capacity (FRC) was also adjustable. (See 
figures 4 and 5. ) 

One of the assets of this machine was 
its anatomically appropriate arrangement 
and functioning of components such as the 
simulated trachea with bidirectional flow 
that provided dead space. Also humanlike 
was the continuous simulated metabolism 
process. In addition, this simulator was 




FIGURE 3.— IBM breathing-simulation system schematic. 



Sample 



To and from 

apparatus 

being tested 




Humidifier- 



Gas analyzers 



/ 



Return 



it 

\ =Jchec! valve 



Reservoir 
Expire rV- 



Metabolic system 



iv V 



' ' • • • • • ' ' • • • • • • • • 



• > > > > > > I > > I I -r 




V 



J 



r 



' * * 'i[ \\ 




Heaters 



O c^ 



Heater Sponges Water 

FIGURE 4.— IBM temperature and humidity system schematic. 



Inspire 
check valve 



Breathing piston 



Controller thermocouple port 

Asbestos end plate \ 

End-plate bolt^^m \ \ 

Moldable 



fiberglass insulation 

Ceramically potted - 
electric heater 



Catalyst beads 



Asbestos . 
end plate 



Combustion tube cap 

Thermocouple 




Electric heater terminal 



Sleeve and 
cap weld 



Electric heater 
terminal 



Combustion tube cap 
FIGURE 5.— IBM electric furnace. 

more of a true closed-loop system (like a 
human being) than simulators that simply 
remove and replace gases to effect metab- 
olism, but this was accomplished not 
without penalty. Control of the furnace 
ignition of the propane was not easy. 
Also, the system was very complicated 
with many modifications such that only 



the person who used and modified it 
understood exactly how it worked. When 
that person left the Bureau, the IBM 
simulator was retired. Only peak values 
of inhaled gases could be measured with 
this system as opposed to average inhaled 
values. 

For more information regarding the 
functioning of this simulator, refer to 
Bureau RI 8496. 2 

REIMERS MBMS 

This machine was bought from Reimers 
Consultants of Falls Church, VA, in the 
spring of 1981 as a supply contract 
(S0308126). It was used in Bureau re- 
search and testing for 2 yr, then it was 
transferred to the NIOSH facility in Mor- 
gantown, WV, for its research projects. 
(See figure 6.) 

The Reimers manual breathing and meta- 
bolic simulator (MBMS) effected metabo- 
lism simulation through cyclical removal 
of breathing circuit gas and replacement 
with C0 2 and N 2 . The N2 replacement was 
made necessary by the unavoidable removal 

^Sparks, A. w. , R. L. Stein, and J. W. 
Stengel. A Breathing Metabolic Simulator 
for Testing Respiratory Protective Equip- 
ment. BuMines RI 8496, 1980, 18 pp. 








<® - » 



o 



I 



f» f> 



WlHllllli 



ill 
wm 




FIGURE 6.— Reimers MBMS photo. 



of some N 2 in the 2 removal process. A 
schematic of the Reimers MBMS is shown in 
figure 7. The flow loop is unidirec- 
tional and contains approximately 7 L of 
breathing circuit gas. Air is inhaled 
from the inhalation port and then mixed 
in the inhalation mixing box from which 
inhalation gas is sampled. The major 
portion of the gas is then inhaled into 
the main bellows. Upon exhalation, the 
gas is forced out of the main bellows 
into the humidifying chamber where mois- 
ture is added to the gas. From this 
chamber, the gas goes into an after- 
heater and an exhalation mixing box from 
which exhalation gas is sampled. The 
motor-driven main bellows is controlled 
by a series of slide potentiometers that 
enable the breath waveform to be shaped. 



Part of the inhaled air is drawn into a 
smaller bellows, called the removal bel- 
lows, to simulate O2 consumption. The 
quantity removed depends upon the concen- 
tration of O2 in the inhaled gas. A 
quantity of gas equal to that removed is 
replaced by the supply bellows consisting 
of both CO2 and the makeup N 2 . An addi- 
tional bellows, called the balance bel- 
lows, is utilized to ensure that the 
operations of the removal and supply bel- 
lows do not add to the desired tidal 
volume. The balance bellows thus serves 
as a volume compensator. 

The metabolism simulation, then, is 
controlled by the supply and removal 
bellows. The quantity of gas exchanged 
through the operations of the supply and 
removal bellows is dependent upon the 



Vent to 
atmosphere 



Inhale port -=. 
RH indicator 




Supply 
bellows 



. COg (constant flow) 
plus N 2 (demand valve) 



7777777/ 

Movable 
To drive fulcrum 
unit 

FIGURE 7.— Reimers MBMS schematic. 



3_A- Exchange 
u ratio control 



concentration of 2 in the breathing 
circuit. If an O2 removal rate of 2 
L/min is desired and the O2 concentration 
is 100%, one simply removes 2 L/ min from 
the circuit. If, however, the 2 concen- 
tration is only 50%, 4 L/min of circuit 
gas must be removed in order to remove 
the 2 L of 2 . 

In a much simpler process, the C0 2 
flows into the supply bellows at a con- 
stant rate and is independent of the gas 
exchange processes. The N 2 flows into 
the supply bellows elicited from a demand 
valve as needed to complete the exchange. 
Thus, at an 2 concentration of 50% (as 
above), and a C0 2 flow rate of 2 L/min, 2 
L/min of N 2 must be supplied from the 
demand valve to equal the 4 L/min of gas 
being withdrawn via the removal bellows. 

The quantity of gas exchanged in the 
metabolism process is controlled manually 



by a knob referred to as the exchange- 
ratio controller. The ratio used is the 
removal bellows volume divided by the 
main bellows volume. Thus, the higher 
the 2 concentration, the lower the ex- 
change ratio. Since this is a manually 
controlled operation, a human monitor 
must carefully observe the 2 concentra- 
tion (measured by the analyzer) of the 
inhaled gases in the inhalation mixing 
box and then adjust the exchange ratio 
accordingly. 

The advantages of this simulator over 
the IBM machine were that it did not use 
combustion of flammable gases to simulate 
metabolism, and that it enabled us to 
measure average inhaled gas concentra- 
tions. This was accomplished mechani- 
cally through its design by using a 
unidirectional flow loop that drew all 
of the inhaled gas from a breathing 



apparatus into the inhalation mixing box 
where the gas concentrations were mea- 
sured. With the IBM simulator, and in 
human subject testing labs, even though 
continual monitoring of gases may be per- 
formed, only minimal and maximal concen- 
trations are utilized. The contribution 
of apparatus dead space to inhaled gas 
concentrations is not calculated if only 
peaks of gas concentrations are noted; 
measuring minimal values of C0 2 , for ex- 
ample, tells only how well the C0 2 
scrubber is working and not how much C0 2 
is actually being inhaled. Average in- 
haled values of gases tell us what con- 
centration of gases a person would actu- 
ally inhale. 

One of the major drawbacks of this 
system was its mechanical complexity. 
Also, since control of the exchange ratio 
was manual, the user was forced to always 
be present during a test because 2 con- 
centration was continually changing. The 
manual exchange ratio also made it prac- 
tically impossible to simulate more than 
one metabolic state. Further problems 
with, the MBMS are next described in 
detail. 

Bellows Flexibility 

Because of the flexible nature of the 
bellows, even though it was reinforced 
with wire, the volume fidelity of 
the system was not always good. If a 
breathing apparatus being tested had 
significantly higher exhalation resist- 
ance than inhalation resistance, for ex- 
ample, the simulator might not be able 
to force the appropriate volume of gas 
back into the apparatus while continuing 
to extract the appropriate volume. This 
would have the effect, in the case of 
closed-circuit breathing apparatus, of 
drawing the breathing bag flat and de- 
manding more 2 than desired. 

N 2 Fidelity 

It is an assumption by design of the 
MBMS that N 2 in the correct quantity will 



be drawn into the circuit to balance the 
2 removal and CC^ addition processes. 
This, however, was not necessarily the 
case. Again because of the flexibility 
of the bellows, if the inhalation resist- 
ance of the apparatus were high and the 
exhalation resistance low, for example, 
more N 2 would be forced into the breath- 
ing circuit. Also, upon every exhala- 
tion, pressure would increase in the sup- 
ply bellows causing the CC^ flow to slow; 
this would reduce the quantity of CO^ 
being added to the system. Since the 
supply bellows demanded that a certain 
quantity of gas be added to the system, 
more N 2 would then be added to make up 
the required volume. Both of these fac- 
tors had the effect of causing (^ concen- 
trations to decrease because the 0^ was 
being diluted by the t^ . 

System Response Time 

Because of the large internal flow-loop 
volume (approximately 7 L) and the uni- 
directional flow pattern, the system re- 
sponse time to a change in inhaled gas 
concentration was longer than that of a 
human being. This had the effect of com- 
promising its simulation. This problem 
would also be carried over to the auto- 
mated version of this design. 

The Hypoxia Scenario 

Also because of the large internal 
flow-loop volume, a problem surfaced with 
the compressed O2 apparatus when 0^ con- 
sumption rate was greater than the con- 
stant C^ flow of the apparatus. Upon 
first attachment of the apparatus, even 
if the simulator were inhaling 100% O2 , 
it would exhale ambient air back into the 
apparatus since the simulator flow loop 
was full of ambient air. This would have 
the effect of diluting the 0^ concen- 
tration and filling the apparatus with 
mostly N2 . Since the 0^ removal rate was 
higher than the 0^ supply rate, the 0^ 
concentration would fall at a constant 
rate until too low for life support. The 



apparatus demand valve would not be trig- 
gered since the large quantity of N 2 kept 
the breathing bag inflated. 

REIMERS ABMS 

This machine was a result of a 3-yr 
development contract with the Bureau. 
This simulator was to automate the design 
of the Reimers MBMS, which was then in 
use. The Reimers ABMS was delivered to 
the Bureau in February 1984. (See figure 
8.) This simulator was also to remedy 
the problems the Bureau had identified 
with the Reimers MBMS. The bellows were 
replaced by flexible rolling-seal pis- 
tons that were stretched to a taut condi- 
tion by pulling a vacuum on their nonsys- 
tem side. It was felt that this design 
change would solve the N 2 fidelity 



problems, but it added another system to 
an already complicated device. See fig- 
ures 9 through 11 for schematics of this 
ABMS. This simulator is described in 
more detail by Reimers. 3 

A supervisory computer was used to con- 
trol the functions of breathing simula- 
tion and to automate the exchange-ratio 
control in order to remove the correct 
amount of gas to simulate 2 consumption. 
Temperature, humidity, and C0 2 flow were 
input and then controlled by computer. 

The advantages of this machine over 
its manual version were its automated 
exchange-ratio control that freed the 

•^Reimers, S. D. The Development of a 
New Automated Breathing Metabolic Simu- 
lator. J. Int. Soc. Respir. Protect. , 
v. 2, No. 1 , 1984, p. 170. 




FIGURE 8.— Reimers ABMS photo. 



10 



t I 



/i 



Rolling seal ^ 

piston assembly 



Piston 



Ldc 



r 




K 



i 



a 



y: 



3 



a 



Jl 

A Vacuum pump 







Rotary absolute 
shaft encoder 



Vacuum switch 




A, 



Waveform 
generator 



w 



Zero- backlash 
ball screw drive 



II W i |i |i|i|i|| i i iii M ii ||i|i|i| i| i [ i|i| i |ffiM 



Point and scale -^ 
assembly for position 



«/ 



L 



Amplifier 



Velocity transducer 
♦ Readout 



Readout 
for position 



FIGURE 9.— Reimers ABMS breathing-simulation system schematic. 



user from being present at all times 
during a test, and its capability to 
simulate more than one metabolic state 
per test. 

The major problem with the Reimers ABMS 
was that it never worked for any length 
of time. At some point during a test, 
for some inexplicable reason, the piston 
would attempt to move beyond range limits 
and would trigger the limit switch, thus 
shutting down the system. The mechani- 
cal system and the computer system were 
designed by different persons, with the 



result that effective control of the 
mechanical system was not achieved by the 
computer system. 

The cause of the problem could not be 
isolated by either Reimers Consultants 
or by the person who created the com- 
puter program through contract to Rei- 
mers. The Bureau then decided to remove 
the computer-control system and run the 
mechanical system from the in-house 
computer. Also, the servo-motor was 
replaced with a stepper motor. The Bu- 
reau is currently attempting to bring the 



11 



Electronic 
indicator 



RO 





^1 


External < 
readouts ' 


4 




1 



-s*- 



Inhale mixing box 



RO i u_ 

"T 1^7^-RHr 



*= n 



A-L 




Test 

manikin 



*=<«= 



T 1 



Drain 



H 

I Gas sample lines 



=5*= 



A/D-D/A \*-^-°-\ ""T^RhI* 

converter ' ^ , 



converter 

and 
supervisory 



SP 



computer RO 

I 






^_ 



Afterheater- 



>, 



JH 



_*Lt£# 



T 



to- from analysis module 



- Afterheater assembly 



• Exhale mixing box 
On -off 



I ^""^7" " — i 120 Vac 



H^ 



t- 



To all temperature 

and relative humidity 

system electronics 



Fan 



Bubble chamber— «■ 



-L_ 



SP 
RO 



11 



^ 



H 2 
temperature 
limit switch 



Fill-drain 
line 



^ 



To AP 

test location 



" Heater 



Liquid level 
sight line 



Indicator-controller 



Main 
air cylinder 



KEY 
RO Readout 
SP Set point 
RH Relative humidity 
T Temperature 
AP Pressure 

FIGURE 10.— Reimers ABMS temperature and humidity system schematic. 



12 




Regulated 
diluent supply 
(user supplied) 



CO2 volume 
compensator 



From CO 2 
add system 



Manual t_ 



—■-I 



FIGURE 11.— Reimers ABMS metabolic-simulation system schematic. 



Reimers ABMS to a working condition so 
that it may be evaluated. 

DEEC INC. ABMS 

This simulator was delivered to the 
Bureau in March 1985; it was modeled 
after its conceptual twin, still in use 
at the Noll Laboratory of Pennsylvania 
State University in State College, PA. 
The DEEC Inc. simulator is a commercially 
available item. The simulator at the 
Noll Laboratory was developed as a 
laboratory tool, part of a Bureau con- 
tract with Penn State. 4 (See figure 12.) 

^Kamon, E., S. Deno, and M. Vercruys- 
sen. Physiological Responses of Miners 
to Emergency. PA State Univ. (contract 
J010092). Volume I — Self -Contained 
Breathing Apparatus Stressors. BuMines 
OFR 29(1) -85, 1984, 32 pp.; NTIS PB 85- 
186831. Volume II — Appendices (contract 
J010092). BuMines OFR 29(2)-85, 1984, 
181 pp.; NTIS PB 85-186849. 



The inventors have described this simula- 
tor in detail. 5 

This ABMS is presently in use at the 
Bureau and is being continually evalu- 
ated. We have found that, due to its 
physical simplicity, there is inherently 
less that can go wrong with it. Its com- 
plexity is in the computer software. The 
breathing simulation is achieved through 
use of a piston attached to a stepper 
motor that is controlled by the computer. 
(See figure 13.) Any shape of waveform 
is capable of being reproduced. At pres- 
ent, one can choose from sine waveforms, 
Silverman waveforms, or waveforms devel- 
oped by Pennsylvania State University 
personnel through a Bureau contract, 
which, in the opinion of the creator of 
the ABMS, are more like those from human 
subjects. 

Water is circulated to a mixing chamber 
on top of the piston from a heated water 

5 Volume II, page 120 of work cited in 
footnote 4. 



13 




FIGURE 12.— DEEC Inc. ABMS photo. 



reservoir. The heated water rains down 
on the piston, humidifying and heating 
the air. The water then drains out from 
the bottom of the piston and returns to 
the reservoir. Metabolism is simulated 
by continual withdrawal and replacement 
of gases through needle valves controlled 
by stepper motors, which are in turn con- 
trolled by the computer. System gas is 
withdrawn from a point in the trachea 
above the lung and through a needle valve 
by a vacuum pump in order to simulate 2 



consumption. If the system gas is 100% 
2 , and an O2 consumption rate of 1 L/min 
is desired, the stepper motor will open 
the needle valve to permit 1 L/min to be 
withdrawn. If the O2 concentration in 
the system is only 50%, 2 L/min of system 
gas must be withdrawn by the vacuum pump. 
The computer measures the system gas 
concentrations through gas analyzers and 
adjusts the needle valve with the stepper 
motor accordingly. 



14 



Heat exchanger 



Valve A 



<3T 



Valve B, 



| Motor -^6>— Motor -^$— Motor -A») 



C0 2 



W 



^ 



w 



Valve C. 



Vacuum 
pump 



J 




Mouth 



Rapid response 

O2 and CO2 

analysis 



Temperature 
measurement 
(4 channels) 



Pressure 
measurement 



Humidity 
water reservoir 



Control data 

* 



Computer 



Disk 
(program and 
data storage) 



X 



Plotter 



Line printer 



Measurement data 



Console 
terminal 



Note: 

1. All motors have direct total control 
from the computer. 

2. Lung assembly is heated to 37° C. 

3. All measurements are available as 
averages or instantaneous. 



FIGURE 13.— DEEC Inc. ABMS schematic. 



Since the system gas rarely reaches 
100% O2 , some N 2 is removed in the pro- 
cess of 2 removal. This N 2 must be re- 
placed and is metered in through another 
needle valve that is also controlled by 
the computer. In addition, C0 2 is added 
to the system to simulate C0 2 production 
by the body through the third needle 
valve. The computer controls all these 
processes. 

A unique feature of this ABMS is its 
ability to electronically measure average 
inhaled gas concentrations. Whereas the 
two Reimers machines measured average 
inhaled gas concentrations by physically 
collecting the inhaled gas into an in- 
halation mixing box that could hold 



several breaths, this ABMS measures aver- 
age inhaled gas levels by integrating the 
area under the inhalation curve of the 
gas tracing, weighted by instantaneous 
flow rate, taking into account gas trans- 
port and analyzer response time. See the 
appendix for a more thorough explanation 
of this concept. 

Other features of the ABMS are 

1. A humanlike bidirectional breathing 
flow path. 

2. Variable rates of 2 consumption 
(0-7 L/min), C0 2 production (0-7 L/min), 
respiratory frequency (6-100 breaths/ 
min), and ventilation (0-130 L/min). 

3. Metabolic rate changes of 4 per 
minute. 



15 



' 4 - 



o 
o 







V 25 7 mln 

Vn= 1.0 '/. 



•DEEC INC. ABMS f 



Human 



min 



I ,/Mi 




Mixing box simulators 
(estimated) 



TIME.s 
FIGURE 14.— Comparison of C0 2 breath waveforms. 

4. Continuous monitoring of average 
inhaled 2 and C0 2 , breathing resist- 
ances, and gas temperatures. 

5. Computer programs for self- 
calibration. 

6. Fast-response gas dryer and ana- 
lyzers for breath-by-breath, average in- 
haled gas concentrations. 

7. Disk storage of complete tests. 

8. Close match between C0 2 exhalation 
waveform of a human subject and that of 
the DEEC Inc. ABMS (fig. 14). 

The metabolism and breathing simulation 
of this simulator overall have proved 
accurate to within 5% of desired values. 

Two possible weak points in the DEEC 
Co. design are the indirect control of 
metabolic flow rates and the sensitivity 
of the electronic calculation of average 
inhaled gas concentration. The metabolic 
flow needle valves are calibrated for 
flow during a 1-h self-calibration pro- 
cedure that is dependent upon steady in- 
let gas pressures for C0 2 and 2 and 
repeatable vacuum pump performance. If 



the inlet gas pressures change or the 
vacuum pump changes its characteristics, 
the metabolic flow rates will be in 
error. 

The calculation of average inhaled 
gas concentrations depends upon correct 
measurement of gas transport and response 
times that are used to delay the integra- 
tion of the area under the figurative gas 
concentration curves. If the gas trans- 
port or response times, which are mea- 
sured in ms , change due to turbulence in 
the sample lines, the measured values 
will be incorrect. Turbulence could be 
caused by discontinuity in the sample 
line if, for example, two lines joined by 
a butt connection become slightly sepa- 
rated and the inner diameter of the sam- 
ple line suddenly expands. Carelessness 
would permit these two weak points to be- 
come problems . 

After further evaluation and familiari- 
zation with this ABMS, the Bureau intends 
to develop recommended revisions to 30 
CFR 11, which details requirements for 
approval of breathing apparatus by NIOSH 
and the Mine Safety and Health Adminis- 
tration (MSHA) . Quantitative evaluation 
of breathing apparatus based upon ABMS 
tests, rather than human subject tests, 
will be recommended. The performance of 
a breathing apparatus should be evaluated 
with a known, controlled input. It will 
be recommended that human-subject testing 
be used only for ergonomic evaluation. 
New stressor levels will be based upon 
recent physiological research. 



CONCLUSIONS 



After more than a decade of Bureau of 
Mines research in the simulation of 
breathing and metabolism, a system has 
been developed that meets the Bureau's 
goals. It can be used as a laboratory 
tool in both research and testing in the 
evaluation of breathing apparatus, espe- 
cially closed-circuit types that cannot 
effectively be evaluated by simple 



breathing machines. This automated 
breathing and metabolic simulator can be 
called upon to produce any waveform or 
metabolic demand that can be produced by 
a human subject, and do it in a repeat- 
able manner that is precisely controlled. 
This tool will be used to better and more 
fairly evaluate breathing apparatus in 
research and testing. 



16 



APPENDIX. —AVERAGE INHALED GAS VALUES 



Some elaboration on the concept of 
average inhaled gas concentrations is 
warranted. When monitoring the gas con- 
centrations sampled at a position close 
to the mouth of a user of a closed- 
circuit breathing apparatus, one would 
observe cyclical changes: high C0 2 and 
low O2 upon exhalation, and low C0 2 and 
high 2 upon Inhalation. A chart record- 
ing of such cycling is shown in figure 
A-l. What is not widely recognized is 
that the low reading of the C0 2 , for 
example, is not the concentration of C0 2 
that is actually being inhaled. This is 
merely the lowest level of C0 2 escaping 
the C0 2 -absorbent canister. 

The column of air being inhaled from 
the breathing apparatus, as shown in fig- 
ure A-2, will contain some exhaled gas 
that resides in the dead space of the 
apparatus. The exhaled gas is high in 
C0 2 and low in 2 ; thus, even though 







1 


1 1 1 1 


1 


1 1 1 




. 



a. 


4 
















- 


~ 


















■ 


O 
O 


2 
















- 






- 






^ 


. 




^. 


• 




1 


1 1 


1 1 


1 


1 


1 1 





58 

a. 
50 



1 1 1 r 



I 




J I I I I I I I L 



I 



I 2 3 4 5 6 7 8 

TIME, s 

FIGURE A-1.— C0 2 and 2 breath waveforms. 



10 




K0 2 
High C0 2 , canister 
Low0 2 




Dead space 

W 

Breathing bag 

Single inhalation air column 
FIGURE A-2.— Single Inhalation air column. 



inhalation has begun, the monitored gas 
concentrations will not change until this 
slug of exhaled air passes through the 
subject's mouth. This must be considered 
in order to determine what the subject is 
actually inhaling. 

A chart recording of the CO2 breath 
waveform, such as in figure A-l, does not 
indicate precisely where inhalation has 
begun. The drop in C0 2 , for example, in- 
dicates the point in time at which the 
air inspired from inside the CO^ scrubber 
and breathing bag has reached the gas 
sampling point, been transported to the 
gas analyzer, and registered on the chart 
recorder. At some point before this time 
on the chart recording, the inhalation 
cycle began. If we knew exactly where on 
the C0 2 recording inhalation began, we 
would have a better idea how much CC^ 
from the dead space of the breathing hose 
was inhaled. Measurement of the instan- 
taneous flow rate would tell us where 
inhalation began, but the response times 
of the gas analyzers must also be known 
for correlation between the CO2 curve and 
the flow rate curve. 

Figure A-3 shows inhalation and exhala- 
tion cycles depicted in three ways: lung 
volume, instantaneous flow rate as mea- 
sured at the mouth, and instantaneous CC^ 
concentrations as measured at the mouth 
reflecting delays of gas transport time 
to the analyzer and analyzer response 
time. In this case, we know exactly when 
inhalation started. If we subtract the 
contributions of the gas transport time 
and the analyzer response time (300 ms) 
on the C^ curve, it would seem that all 
we need do is integrate the area under 
the curve from that point when inhalation 
begins until inhalation ends and exhala- 
tion begins. Exhalation can be assumed 
to begin 300 ms (gas transport and ana- 
lyzer response time) after the point in 
time indicated by the lung volume cycle. 
The further delay in CO2 rise, after the 
gas transport and analyzer response times 
have been accounted for, is due to the 
low C0 2 found in the tracheal dead space. 



Another factor must 
ever, before actual 
concentrations can 



be considered, how- 
average inhaled gas 
be determined, and 



17 



that is the fact that the breathing flow 
rate is changing all the while the gas is 
being sampled. If, for example, a high 
C0 2 concentration is registered over a 
100 ms time period while the flow rate is 
high, it indicates a greater quantity 
of C0 2 being inhaled than the same con- 
centration over the same time interval 
while the flow rate is low. Therefore, 
each instantaneous CO2 reading must be 
weighted by multiplying it by its 
corresponding instantaneous flow rate; 
each of these products are then added 
together in order to determine how much 
gas is actually being inhaled. 

With a breathing and metabolic sim- 
ulator such as the DEEC Inc. ABMS, 
it is easily determined when inhalation 
begins. The computer knows when it gives 
directions to the stepper motor to start 



inhaling. The computer also knows what 
the instantaneous flow rate is because of 
the defined relationship between stepper- 
motor speed and the fixed-diameter pis- 
ton; this determines flow rate. During a 
calibration procedure performed before 
each test, the DEEC Inc. ABMS measures 
the gas transport and analyzer response 
times. Then, knowing what these time 
delays are, it multiplies the instan- 
taneous flow rates by the correlated 
instantaneous gas concentrations and adds 
them up over the inhalation cycle in 
order to determine the quantity of CO2 
or O2 inhaled. This is divided by the 
calculated tidal volume to get a truly 
accurate average inhaled gas concentra- 
tion measurement. On the DEEC Inc. ABMS, 
these values are determined every two 
breaths. 




o 

Q. 

o 
o 



10 

~8 

6 

4 

2 



1 

Li 



1 1 1 

— H H-Gas sample transport and analyzer response time 
■*- C0 2 due to apparatus dead space 

-HH*-C0 2 due to tracheal dead space 

U-C0 ? escaping CO? scrubber 
KEY 
Y7\ Av inhaled 
C0 2 



'/>»>/?/>J»V>?l 




Minimal C0 2 
value 



I [£2gzzzzzzzz2zzz! 



2 4 

TIME t s 

FIGURE A-3.— Minimum C0 2 value vs average inhaled C0 2 value. 



8 



U.S. GOVERNMENT PRINTING OFFICE: 1986—605-017/40.099 



INT.-BU.0F MINES,PGH.,PA. 28372 



« 29 



U.S. Department of the Interior 
Bureau of Mines— Prod, and Oistr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh, Pa. 1S236 



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